Nowhere near Leah... She was a clear psychopath, no doubt, it was like just out of example books. Sage is a rather difderent story. She is from an important family, leader of a sorority, so she has quite the reputation. If she wouldn't have deep feelings she would be furious for being cheated on as well by that reasons, she can not afford nor tolerate not to be NO. 1. That is why she handles Chad's stuff really bad as it hurts not just her clearly present feelings, but her overall outer reputation too. She wants revenge, sure. But she takes it to "just" give the similar treatment she received.
And let's face it: if either of us suspect our partner cheating we would be also cueious and try to seek thw truth. Only the tools differ for it by altering personalities. She only "stalks" to that point to be sure about what Chad do behind her back and only considering their relationship. And she even asks for help with that, not something a person with criminal instincts would do.